Just tested some of my intranet pages where I have fieldsets with applied CSS style overflow:hidden for clearing floating elements inside fieldset – and in Opera 9 TP1 it doesn't clear anything and do weired things.
Just one more thing to make it broken – add position:relative in addition to overflow:hidden.
This bug appears to be not connected to only fieldsets it also "works" with DIVs too.
I also create separate test page for it.
Also notice the missing top border on the legends for fieldsets, and weird position of header "DIVs".
This test page also shows bug fixes in Opera 9 related to margins (there should be no bottom margin under first fieldset and first div). :up: